Selective catalytic reduction (SCR) systems have been used to reduce automotive emissions. Such systems typically add a gaseous or liquid reductant, such as ammonia or urea, to the exhaust gas stream from an engine to be absorbed onto a catalyst where the reductant reacts with nitrogen oxides in the exhaust gas to form water vapor and nitrogen. However, SCR systems may rely on uniform mixing of the reductant with nitrogen oxides in the exhaust stream upstream of the SCR catalyst. This may lead to various problems. For example, ineffective mixing of urea delivered from an on-board storage device may reduce the conversion efficiency of nitrogen oxides at the SCR catalyst.
One way to facilitate mixing of urea injects a urea solution directly into the engine exhaust stream. For example, an injector may apply pressure to force the urea solution through an orifice to facilitate a dispersion of urea in the exhaust stream. However, concerns of equipment weight and robustness against orifice plugging may prove such systems undesirable.
The inventors herein have realized that these concerns and others may be addressed by applying an ultrasonic field to a reductant-containing liquid in the reductant storage device to generate a mist of the reductant-containing liquid, and selectively introducing an amount of mist to an exhaust system coupled to the catalyst based on an engine operating condition. Such methods of generating and supplying a mist from the reductant may facilitate the delivery of a uniform mixture of reductant and nitrogen oxide and nitrogen dioxide (NOx) to the emission control device with reduced weight and robustness concerns.